1) Field of the Invention
The present invention relates to an aromatic polyimide double layered hollow filamentary membrane and a process for producing the same. More particularly, the present invention relates to an aromatic polyimide double layered hollow filamentary membrane having an excellent gas-permeating and separating property, heat resistance, chemical resistance, and pressure resistance and useful as a reverse osmosis membrane, ultrafiltration membrane, gas-separating membrane, for example, hydrogen-separating membrane, carbon dioxide-separating membrane or oxygen-concentrating membrane, pervaporation membrane, organic vapor-dehydration membrane, and dehumidification membrane, and a process for producing same at a high reproductivity and efficiency.
2) Description of the Related Arts
Various types of separation membranes are currently utilized for energy saving in the technical fields of separation, concentration, and refining of substances, and the importance of this technology is growing.
There have been many attempts to provide separating membranes, by various methods, and the known separation membranes are usually composed of a separating resinous layer for separating a substance and a supporting layer for imparting a mechanical strength to the membrane.
The known methods of producing the separating membranes can be classified into two groups:
1) A method in which a solution of a film-forming polymer is spread to form a thin layer of the solution, and the solution layer is solidified by a drying-coagulating method. In this method, a dense separating layer and a porous supporting layer are simultaneously formed to provide an asymmetric membrane.
2) A method in which a pre-formed porous membrane is used as a supporting layer and then a dense separating layer is formed on a surface of the supporting layer to provide a composite membrane.
U.S. Pat. No. 4,460,526 discloses a process for producing an asymmetric separating membranes from an aromatic imide polymer, and the resultant membranes exhibit a high heat resistance, chemical resistance, and gas-permeating property.
The conventional hollow filamentary membranes having a satisfactory gas-permeating property are often unsatisfactory due to a poor pressure resistance thereof.
In conventional processes for producing a hollow filamentary membrane, a single spinning solution of a polyimide is used to simultaneously form both the dense separating layer and the porous supporting layer united with the separating layer. Accordingly, it is very difficult to provide a dense separating layer and a porous supporting layer, respectively, having the necessary satisfactory properties for effecting desired functions, from the one and the same polyimide spinning solution by the one and the same spinning procedure. Namely, the type and concentration of the polyimide in the dope solution and the spinning conditions are very restricted; e.g., sometimes the resultant hollow filamentary membrane exhibits unsatisfactory separating and permeating properties, or sometimes the spinning operation requires the expertise of a person specifically skilled in the art. Also, sometimes it is difficult to stably produce the hollow filamentary membrane at a satisfactory reproductivity.
U.S. Pat. No. 4,690,873 discloses a process for producing an asymmetric gas-separating membrane from an asymmetric biphenyltetracarboxylic acid type aromatic imide polymer. In this process, a dope solution is produced by dissolving a solvent-soluble aromatic imide polymer produced from an aromatic tetracarboxylic acid component comprising, as a principal ingredient, a biphenyltetracarboxylic acid and an aromatic diamine component comprising, as a principal ingredient, an aromatic diamine compound having a divalent radical of the formula --SO.sub.2 --, in an organic solvent, and the resultant dope solution is subjected to a wet membrane-forming method in which a thin layer of the dope solution is solidified in a coagulating liquid, to form an asymmetric gas-separating membrane, which may be in the form of a hollow filament, consisting of a dense outer surface layer and a porous inner layer. The resultant aromatic polyimide gas-separating membrane exhibits an excellent heat resistance and chemical resistance, and a satisfactory carbon dioxide gas-permeating property.
The methods of forming the dense separating layer in a composite membrane are as follows:
(1) a solution of a film-forming polymer in a low concentration is coated in a small thickness on the supporting layer and the thin layer of the polymer solution is dried;
(2) a solution of a film-forming polymer is spread on a water surface, the spread thin layer of the solution is solidified, and the resultant thin membrane is laminated to the supporting layer; and,
(3) a solution of a monomeric material is coated on a surface of the supporting layer and the monomeric material is polymerized to form a polymeric membrane on the supporting layer.
In conventional processes for producing the hollow filamentary composite membrane disclosed in U.S. Pat. No. 4,528,004, a thin dense coating layer of a polyimide is formed around a peripheral surface of a porous hollow filamentary membrane, but this coating operation must be carried out under a very accurate and complicated control, to avoid an undesirable formation of pinholes in the coating layer. Therefore, it is difficult to form a thin separating layer having a uniform thickness and substantially no defects such as pinholes on the porous hollow filamentary membrane, at a high reproductivity.
Japanese Unexamined Patent Publication (Kokai) Nos. 49-62,380, 62-191,019 and 64-15,104 disclose multilayered hollow filamentary membranes and processes for producing same from polymeric materials, for example, cellulose acetate, and polyamide.
The conventional multilayered hollow filamentary membranes have an unsatisfactory heat resistance, chemical resistance, and pressure resistance.
In a practical energy-saving gas-separating process, however, the conventional aromatic polyimide gas-separating membrane does not always have a satisfactory pressure resistance and gas-permeability.
Accordingly, there is a strong demand for the provision of a hollow filamentary gas-separating membrane having a high pressure resistance and a satisfactory gas-permeability.